Can Our Hearing Be Damaged By Sounds We Can’t Hear?

Possibly. We hear roughly 20 Hz to 20,000 Hz, but intense low-frequency sound below that range can briefly change the inner ear, causing fluctuations in our spontaneous otoacoustic emissions that leave the ear more vulnerable. These changes are temporary, and there is no clear evidence that everyday inaudible sound causes permanent hearing loss.

When a firetruck blares past us on the street, or if we get jostled to the spot directly in front of the speakers at a concert, the noise is not only annoyingly loud, but it can also damage our hearing. We instinctively avoid situations like this, as they are uncomfortable and potentially painful, but what about sounds we can’t hear? In the same way that our eyes are only able to see light in certain wavelengths (visible light), our ears similarly have a range within which we can hear. Outside of that range, it becomes much harder to determine if something is dangerous to hear.

Before we dive too deeply into the possible risks of exposure to sounds outside our accepted range, we should take a quick look at our ears in a bit more detail.

Hearing And The Human Ear

When we are exposed to a sound in the range we can detect, which is 20Hz-20,000 Hz, those sound waves pass into our ear canal and strike our eardrum, which will then pass those vibrations to tiny bones of the middle ear, and then into the cochlea (the inner ear structure). Here, those vibrations are picked up by thousands of tiny, specialized hair-like structures that can transfer the vibrations to chemical signals, and then communicate those signals to the brain, which generates the pleasure of sound!

Now, while this is a perpetual and nearly instantaneous transaction between stimuli and reaction, it is incredibly complex and the necessary parts of the ear are very delicate. Those small and sensitive hairs can be damaged while listening to sounds that are too loud or too high-pitched. The hairs are “tuned” to different frequencies, covering the entire range of frequencies that we can hear. If those small hairs are bent or broken, they will not grow back, which means that your range may shrink. The hairs for detecting the highest frequencies are particularly delicate, and are most prone to damage, which is why some people are unable to hear higher-pitched sounds. As more of these hairs become bent, damaged or broken, they will require a greater stimuli to respond to sound (i.e., you will need to crank the volume up even higher).

Furthermore, as you age, your ear hair sensitivity (if you will) diminishes, making it harder to hear the highest edges of the audible spectrum. The textbook hearing range for humans is 20-20,000 Hz, but that upper limit drops fairly quickly with age. Even healthy young adults rarely hear all the way to 20 kHz, with most topping out somewhere around 16,000-17,000 Hz, and by middle age the ceiling often falls below 15,000 Hz. The lowest audible frequencies, near 20 Hz, hold up far better over a lifetime.

What many people don’t realize is that the damage does not stop the moment the noise does. When those hair cells in the cochlea are overworked, they generate a surge of reactive oxygen species (unstable, oxygen-based free radicals). These can keep harming nearby cells for hours or even days after the original exposure, which is part of why hearing can keep deteriorating after a single loud event. This is an active area of research: antioxidant compounds such as D-methionine have protected against noise-induced hearing loss in animal studies, but they remain experimental and are not an approved over-the-counter remedy you can simply pick up to protect your ears.

The Sounds We Can’t Hear

Extremely loud noises or extended periods of exposure to the lowest and highest frequencies we can hear can cause damage, but that doesn’t answer our original question.

By their very nature, sounds we can’t hear don’t have much of a purpose for humans, but there are some instances where we are exposed to sounds beyond our capacity. A jet engine is unmistakably loud, but alongside that audible roar it also pumps out powerful low-frequency and infrasonic energy that sits below what we consciously register, and the dull rumble of a packed stadium can reach into the same range. Even elephants generate extremely low-frequency calls (at very high decibel levels) as a way of communicating over long distances.

Dog whistles are a tool that we’ve created in order to summon our dogs from long distances, but the human ear cannot detect the sound at such a high frequency. You can blow into that whistle as hard as you want, but you won’t be able to hear the sound being generated. Now, these “out of frequency” sounds don’t arise that often in our daily lives, but they are out there, which begs the question of their potential risk.

While the research on this subject has been somewhat limited, a number of studies have turned up some surprising results. When we expose our ear to very low-frequency sound, it can temporarily disrupt the normal functioning of the ear. More specifically, a 2014 German study (Kugler and colleagues at the University of Munich) placed participants in a soundproof booth and exposed them to a 30 Hz tone for about 90 seconds. This was not a faint sound, it was played at 80 dB(A), which is a high sound-pressure level at such a low pitch, even though 30 Hz sits right at the bottom edge of what we can consciously hear. After the exposure, the spontaneous otoacoustic emissions of the participants’ ears were measured. For those who don’t know, spontaneous otoacoustic emissions are generated constantly by a healthy ear, although we can’t hear them. These emissions are like a tiny internal whistle, faint enough that they can only be picked up with a sensitive microphone placed in the ear canal.

In a normal situation, the spontaneous otoacoustic emissions (SOAEs) should stay fairly constant in magnitude, but exposure to high-decibel noise is known to make them fluctuate. In the German study, the 90 seconds of low-frequency sound made the participants’ SOAEs slowly rise and fall in strength for a couple of minutes afterward, an effect the researchers nicknamed the "Bounce." Crucially, these shifts were temporary and recovered on their own, and they are not the same thing as hearing loss. What they do suggest is that, for a short window after intense low-frequency exposure, the cochlea is working differently and may be a little more vulnerable than usual.

Researchers would still like to know what happens after much longer exposure to inaudible low-frequency noise, but that question is hard to study directly because of the risk it poses to volunteers. As mentioned above, the high-frequency hairs in the cochlea are also the most sensitive, so pushing their limits isn’t the most viable means of study. However, the tympanic membrane in the ear is believed to filter out noise that is of extremely low and high frequencies, meaning that certain “silent noises” may never even reach the cochlea, let alone damage the hair cells found there.

A Final Word

Learning about potential damage to cochlear hair cells may require putting test subjects or study participants at risk of hearing loss or damage. While understanding these phenomena is important, academic research must be carried out in a responsible manner. Being exposed to low- and high-frequency sounds that we cannot hear is not a common occurrence, and is unlikely to pose a risk to you. However, depending on your job, hobbies or lifestyle, you may be putting yourself at risk in some way.

The best thing to do is take care of your ears when it comes to sounds in the audible range, and avoid hanging around too many jet engines without proper ear-protection equipment!